The present invention relates to the field of combined voice and data communications systems and more particularly to an integrated line card for coupling telephony and high-rate data communications functions to a two-wire telephone loop.
With the increasing popularity of the Internet, there has been a corresponding increase in the demand for high rate digital transmission over the local subscriber loops of telephone companies. A loop is typically a twisted-pair copper telephone line coupling a user or subscriber telephone to a switching station such as a central office (CO).
Conventional data communication equipment uses the voice band of the subscriber loop. Such equipment includes voice band modems, which operate at up to 56 kbps. On the other hand, Integrated Services Digital Network (ISDN) systems have enabled greater data rates over existing copper phone lines 128 kbps. However conventional voice band equipment is limited by the maximum data rate of the existing switching networks and PCM (Pulse Code Modulation) data highways.
Utilizing the frequency bandwidth of the loop outside the voiceband, other high-speed systems have evolved. However because loops can differ in distance, diameter, age and transmission characteristics depending on the network, they pose some limitations and challenges for designers.
Current high-speed digital transmission systems of the above type include asymmetric, symmetric, high-rate, and very high-rate digital subscriber loops, conventionally known as ADSL, SDSL, HDSL and VDSL respectively. Normally these and other similar protocols are known as xDSL protocols.
Of these types of xDSL, ADSL is intended to co-exist with traditional voice services by using different frequency spectra on the loop. In the future, it is possible that multiple transmission schemes may be employed in different frequency bands on the same loop, and that these transmission schemes may include traditional analog voice services as well as current and new forms of xDSL. In known ADSL systems, the so-called "plain old telephone services" (POTS) is allocated the frequency spectrum between 0 kHz and about 4 kHz and ADSL is allocated the frequency spectrum between 30 kHz and 1.1 MHz for data over the telephone line. This is shown schematically in FIG. 1A. ADSL also partitions its frequency spectrum with upstream (subscriber to CO) transmission in a lower frequency band, typically 30 kHz to 138 kHz, and with downstream transmission in a higher frequency band, typically 138 kHz to 550 kHz or 1.1 MHz. ADSL uses a discrete multi-tone (DMT) multi-carrier technique that divides the available bandwidth into approximately 4 kHz sub-channels.
The architecture, interfaces and protocols for telecommunications networks incorporating ADSL modems are illustrated in FIG. 1B. The elements consist of one or more ATU-Cs 2 (ADSL transmission units or ADSL modems) at a central office end 4. The ATU-C 2 can be integrated within an access node 6, which is the concentration point for broadband data 8 and narrowband data 10. Broadband and narrowband in this context is meant telecommunications systems for data rates above 1 Mbps and telecommunications systems for data rates at or below 1 Mbps, respectively. The access node 6 can be located at a central office or at a remote site. A remote access node may also subtend from a central access node. The ATU-Cs 2 are, in conventional technology, coupled via a splitter 12 to the telephone loop 14. The loop 14 at the customer end is also coupled via a splitter 12 to an ATU-R 16 or an ADSL modem which may be integrated within an SM (Service Module), which are devices that perform terminal-adaptation functions. Examples of SMs are Set Top Boxes, PC interfaces, or LAN routers. The PSTN (Public Switched Telephone Network) 18 to the subscriber phones 20 shares the loop 14 via the splitter 12, which isolates the POTS from the ADSL modems.
The splitter 12, or POTS splitter, is the standard model for the deployment of xDSL services and POTS services onto the same physical copper pair 28. An analog POTS splitter 12 is shown in FIG. 2 and provides the filtering required to separate the POTS and ADSL bands on the copper pair before being input to their respective transceivers 23 and 21. The POTS splitter 12 is bi-directional and is also used to combine the POTS and ADSL bands onto the same copper pair 28. Generally, the POTS splitter 12 consists of a low pass filter (LPF) 36 inserted between the telephone interface and the loop and a high pass filter (LPF) 38 inserted between the ADSL transceiver and the loop. The LPF 36 passes the voice signals and the HPF 38 passes the xDSL signals.
It is usually assumed that the key function of the POTS splitter 12 is to separate high frequency (ADSL) and low frequency (POTS) signals at the network end and premises end to pass to their respective line cards. The actual function is more complex and deals with the need to provide the correct impedance on the line in different frequency bands in order to allow signals to properly propagate along the loop and meet the relevant specifications and standards.
In the case where separate POTS equipment and xDSL equipment are used, the low pass function of the POTS splitter serves a second function. Not only does the POTS splitter split off the voice signals to the POTS but it also eliminates any high frequency signals that may emanate from the POTS equipment (such as relay switching transients) and which would otherwise interfere with the xDSL signals. In particular, with legacy POTS equipment, these signals are more likely to be produced and therefore need to be filtered. It is desirable however to eliminate such switching transients and inband interferences without adding extra low pass or high pass filters in the signal path.
Further disadvantages of the analog splitter are bulk and expense. Thus a number of solutions have been proposed to eliminate the splitter. For example, U.S. Pat. No. 5,757,803 describes a POTS splitter assembly with an improved trans-hybrid loss for xDSL transmission. U.S. Pat. No. 5,889,856 describes a system for using a digital splitter rather than an analog splitter. In this implementation, the loop is coupled to a single analog-to-digital (A/D) converter. The waveforms from the analog phone line are converted to digital values by the A/D converter, and the digital splitter separates the low frequency POTS from the high frequency ADSL. Although this patent describes an architecture that is conceptually feasible, it is technically limited. Specifically, it is assumed that the front end of the A/D converter is able to adequately match the impedance on the loop at different frequencies in order for the xDSL and voice signals to properly propagate across the loop. Although this patent describes an attempt to eliminate the analog splitter, some form of analog filter must be implemented in its analog line interface in order to allow the A/D to work properly. These analog filter components as described earlier are rather bulky and accordingly do not achieve the desirable advantage to eliminate the splitter and its associated disadvantages.
A further problem faced by designers of integrated line cards is caused by the effect of signal frequency on the loop characteristics. Typical twisted pair loops designed for voice telephony exhibit complex impedances in the voiceband and tend to exhibit purely resistive-type characteristic impedance as the frequency of the propagating signal increases, as depicted in FIG. 3. When ADSL signals are added to a POTS loop they are typically added at frequencies well above the voice band where the loop impedance more closely approximates the resistive characteristic impedance. In the case of both the POTS signal and the xDSL signal the design goal is to generate minimal signal reflections back from the loop onto the card. This is especially true in the case of the POTS voiceband where the transmitted signal and the receive signals share the same spectrum. This is also true in the case of "echo cancelled" xDSL signals, where part of the downstream and upstream spectrums overlap.
The above is achieved in the case of separate xDSL and POTS circuitry by matching the drive and termination impedances as closely as possible to the loop impedance in the frequency band of interest. Thus when either a POTS line card or a xDSL line card is present on the loop alone, the line card (or xDSL modem) is capable of terminating the loop at the appropriate frequency dependent impedance. The POTS or xDSL card is also equipped with all the required filtering needed to eliminate out of band signals and interferences. Furthermore, no external filters such as those used in POTS splitters are needed in the single termination case.
In contrast, when both a POTS signal and a xDSL signal are bridged together without a POTS slitter on a loop, the impedance seen by the xDSL signal is the desired impedance on the loop in parallel with the impedance on the POTS line card. Similarly, the impedance seen by the POTS signal is the desired impedance on the loop in parallel with the impedance on the xDSL line card. In this case, a POTS splitter would not be required if, for example the POTS line card and the stub of cable used to bridge the card onto the loop appeared as a very high impedance compared to the loop impedance at xDSL frequencies, and conversely the xDSL line card and the stub of cable used to bridge it onto the loop appeared as very high impedance compared to the loop impedance at POTS frequencies. Furthermore, it is specified that neither the POTS line card nor the xDSL line card transmit interfering noise in either the xDSL band and the POTS band respectively. Finally, the respective analog front ends (AFEs) of the POTS line card and the xDSL line card must be guaranteed not to overload due to the signal energy of the xDSL signal and the POTS signal respectively. Further the band pass function normally present in the POTS line card should reject the out-of-band energy from the xDSL line card and similarly the high pass function normally present in the xDSL line card should reject the out-of-band energy from the POTS line card.
In practice the above conditions are difficult to meet since the impedance of legacy POTS equipment at xDSL frequencies is uncontrolled and differs with different implementations. Moreover, the length of stubs used to bridge the xDSL and POTS signals onto the loop are difficult to control and if used would add complex frequency dependant impedances. In addition, legacy POTS equipment transmit noise signals that are in the band of the xDSL signal. As a result, it is conventional wisdom that a POTS splitter is required when bridging legacy POTS line cards and new xDSL line cards onto a loop.
Accordingly, one aspect of the present invention is to mitigate some of the disadvantages associated with current integrated xDSL and POTS equipment as described above.